COMPETITION BETWEEN SOLVATION AND INTRAMOLECULAR HYDROGEN-BONDING IN MICROSOLVATED PROTONATED GLYCINE AND β-ALANINE

Infrared predissociation (IRPD) spectroscopy is used to reveal and compare the microsolvation motifs of Gly\ce{H^+(H2O)_n} and $\beta$-\ce{AlaH^+(H2O)_n}. The chemical structure of these amino acids differ only in the length of the carbon chain connecting the amine and carboxyl terminals, which nonetheless leads to a significant difference in the strength of the intramolecular C=O$\cdots$H--N hydrogen bond in the unsolvated ions. This difference makes them useful in our studies of the competition between solvation and internal hydrogen bonding interactions. Analysis of the IRPD results reveals that the sequential addition of water molecules leads to similar effects on the intramolecular interaction in both \ce{GlyH^+(H2O)_n} and $\beta$-\ce{AlaH^+(H2O)_n}. Solvation of the -\ce{NH3^+} group leads to a weakening of the C=O$\cdots$H--N hydrogen bond, while solvation of the carboxyl -\ce{OH} leads to a strengthening of this bond. Additionally, we have found that for $\beta$-\ce{AlaH^+}, the addition of a \ce{H2O} to the second solvation shell can still influence the strength of the C=O$\cdots$H--N hydrogen bonding interaction. Finally, because the C=O$\cdots$H--N interaction in $\beta$-\ce{AlaH^+} is stronger than that in \ce{GlyH^+}, more solvent molecules are needed to sufficiently weaken the intramolecular hydrogen bond such that isomers without this bond begin to be energetically competitive; this occurs at $n = 5$ for $\beta$-\ce{AlaH^+} and $n = 1$ for \ce{GlyH^+}